1. Field of the Invention
The present invention relates generally to nondestructive tire testing and, more particularly, to the use of ultrasound for nondestructive tire inspection and testing.
2. Description of the Prior Art
The tire industry has recognzied for many years that it is useful to inspect or test tires in a nondestructive manner in order that defects in the tires may be readily identified and located. This need has been recognized both for new tires, prior to their initial use on automobiles or other vehicles, and for used tires, prior to retreading. Thus, it is desirable to detect flaws in the tire when the tire is complete with its tread. Various technologies have been suggested in the past to achieve these desireable results, but none has achieved substantial commercial acceptance. For example, halography has been used and can detect both holes and internal defects, such as ply separations, but it requires very expensive equipment used by operators who have been specially trained. Also, high voltage arc equipment has found limited application, but this equipment does not detect internal tire defects. Finally, ultrasound equipment has been used extensively, but it too suffers from a number of disadvantages, including the difficulty of obtaining accurate tire defect data and/or interpreting the test results which in some prior art equipment is presented on an oscilloscope.
U.S. Pat. Nos. 4,266,428, 4,285,235 and 4,275,589 disclose an apparatus and method for nondestructive testing of tires with ultrasound. More specifically, these patents teach the use of pair of ultrasonic transmitters having a beam width of 90.degree. that are positioned within a tire that preferably has been buffed, i.e. had the tread pattern ground off in preparation for retreading. The apparatus retracts the transmitters into a central hub during installation and removal of the tire and extends them into the tire during testing. These transmitters produce ultrasound of a moderately high frequency, e.g. greater than approximately 40 KHz and, preferably, 75 KHz. The ultrasound rom these transmitters is applied to wall of a tire inflated to approximately 15-18 pounds per square inch ("PSI"). Numerous ultrasonic receivers are arranged about the tire's outer surface to receive ultrasound from one or the other of the transmitters. In addition to receiving ultrasonic signals from the transmitters within the tire, these receivers can also detect air leaking through the pressurized tire.
In testing a tire with ultrasound, the apparatus disclosed in these patents operate in a pulse burst transmission mode in which only a few periods (e.g. 30-100) of the acoustic waveform are applied to the tire. According to these patents, the use of pulse burst transmission allegedly reduces standing waves in the tire or unwanted reverberation effects. The two transmitters, located inside the inflated, revolving tire, are energized separately allegedly to prevent distortion of readings from peculiar wave cancellation, standing wave patterns or similar wave effects.
These patents also disclose that the electrical signal produced by the receivers passes through a gated receiver circuit so that only those signals within the initial portion (e.g. the first 10 cycles) of each pulse burst are used in testing the tire. According to these patents, using only the initial portion of each pulse burst reduces alteration of the envelope of the received acoustic signals by internal reverberation, standing wave, wave cancellation or other wave effects.
The gated receiver circuit included in each signal processing channel for each receiver includes an Automatic Gain Control ("AGC") amplifier. These patents disclose that the AGC amplifier is required to compensate both for differing wall thicknesses within a single tire, and for differing thicknesses between tires. The gated receiver also includes a rectifying circuit and an integrating circuit to average the signal over several cycles during the beginning of each pulse burst. To further improve the signal-to-noise ratio, the apparatus may also include a non-linear Analog-to-Digital Converter ("ADC") that digitizes the output signal from the integrating circuit.
These patents also teach that it is advantageous to test the tire with pulsed ultrasound at two different frequencies and to combine output signal obtained at these two different frequencies to avoid possible adverse standing wave effects and the like.
Despite the use of complex signal processing circuits and techniques in the apparatus disclosed in these patents, commercially available versions exhibit operational difficulties which limit their use. For example, the apparatus is extremely sensitive to noise from surrounding equipment. Accordingly, it is often necessary to isolate the apparatus in a special "quiet" room separated from other tire processing equipment. While the apparatus disclosed in these patents provides a visual oscilloscopic display of the testing results, reliably rejecting defective tires while not rejecting good ones requires observation and analysis of the display by an experienced and highly skilled operator. Also, this apparatus cannot reliably test treaded tires, i.e. tires that still retain a tread pattern and that are thicker than a buffed tire. Because these prior art products require buffing prior to ultrasonic testing, the time and cost of buffing a tire must first be expended before it may be discovered that the tire is defective and cannot be retread.